In general, positioning techniques fall into two main categories, navigation and surveying. Traditionally, surveying has required greater precision and thus used differential techniques requiring long periods of observation, whereas navigation has required less precision and emphasized real-time position determination. The development of real-time kinematic (RTK) positioning has resulted in systems that are widely applied to both surveying and navigation.
As shown in Prior Art FIG. 1, an RTK positioning system typically includes a constellation of satellites 105, a base or reference station 110, and a rover 115. The constellation of satellites 105 nominally has N satellites, however, the number of satellites that will be broadcasting signals that are available to reference station 110 and rover 115 will vary.
For purposes of this disclosure, a satellite refers to any fundamental source of raw position data such as that transmitted by the GPS, GLONASS, or planned Galileo orbiting satellites, or an earthbound source (e.g., pseudolite). A satellite positioning system (SATPS) refers to a system using either extra-terrestrial satellites or terrestrial satellites (pseudolites) or a combination as sources of raw position data.
In RTK positioning, centimeter level accuracy is obtained by having both the reference station 110 and the rover 115 track the carrier phases of the same satellite(s) at the same time, using antenna 111 and 117, respectively. High accuracy position determination is achieved by applying a mathematical model for relative positioning (e.g., double-difference model) to the simultaneous measurements of the reference station 110 and rover 115. The double differences essentially eliminate common mode errors (e.g. clock errors), and can be processed to produce a precise baseline (dx, dy, dz) between the reference station and rover.
When the reference station position is accurately known in a given coordinate frame, the rover position can also be determined in the same frame. Even if the reference station position is not accurately known, precise relative positioning can still be carried out.
The key to the real-time aspect of RTK positioning is communication between the reference station and the rover. The data collected by the reference station and the data collected by the rover is combined at a single location for processing and position determination. For RTK, this data combination is usually done by wireless communication, and the data may be combined at the reference station, rover, or other location for processing.
In the example of Prior Art FIG. 1, data is transmitted from the reference station 110 and received at the rover using antenna 112 and antenna 116, respectively. It is the immediate communication between the rover 115 and reference 110 station that enables real-time determination of relative or absolute position, velocity and time.
Historically, the link between a reference station and a rover has been made using a transmitter at the reference station and a receiver at the rover. Correction data from a reference station may be formatted according to various proprietary or published formats, e.g., the Radio Technical Commission Marine (RTCM) format or the Trimble CMR/CMR+. For RTK using double differencing and the RTCM format messages 18 and 19, data is updated about every 0.5 to 2.0 seconds. For RTK, data rates of between 2,400 bps and 9,600 bps are commonly used.
Since the accuracy and reliability of integer ambiguity resolution degrades with increasing separation between the reference station 110 and the rover 115 of FIG. 1, more than one reference station may be networked in order to provide increased rover range and more accurate corrections.
Prior Art FIG. 2 shows an example of a networked RTK positioning system 200. Two reference stations 210 are coupled together so that the SATPS data from both reference stations may combined at control center 215 to provide correction data for a rover 220. The control center 215 may also serve as a reference station, in which case the network would have three reference stations.
To aid in minimizing the amount of data that must be transmitted, one of the reference stations may be designated as a master reference station. The transmitted data to the rover would include the correction data for the master reference and the correction differences for each of the other reference stations in the network.
The preparation and formatting of the transmitted data is performed by the control center 215, and the formatted data is forwarded to the transmitter 220 for broadcast to the rover. A discussion of correction data transmission in a networked RTK positioning system may be found in “A Novel Approach for the Use of Information from Reference Station Networks Conforming to RTCM V2.3 and Future V3.0.,” Zebhauser, B. E., H.-J., Euler, C. R. Keenan, G. Wübbena; presented at the National Technical Meeting, ION NTM-02, Jan. 29-30, 2002, San Diego, Calif.
Current RTK systems often have a dedicated wireless link. Frequencies from 150 to 174 MHz in the VHF band, and from 450 to 470 MHz in the UHF band may be licensed for RTK radio links, as are other frequencies that must be shared with other uses. Narrow-band FM is typically used.
Due to the typical data rate requirements for the transmission of RTK data, low-band VHF (frequencies between 30 MHz and 88 MHz) has been ignored for data transmission in RTK systems, and the trend has been to move to higher frequencies as wireless technology has advanced. However, the higher frequencies are becoming more congested as wireless communications usage increases.
Although the low-band VHF frequencies have been ignored for use in RTK positioning systems communications, they have long been used for other applications such as paging services.
Prior Art FIG. 3 shows an example of a paging system 300. The system elements shown are not all required, and may be included or omitted from different paging systems. A paging terminal 325 is connected to message sources 310, 315, and 320. The message sources provide access to the paging system subscribers.
The paging terminal 325 is the entry point to the paging system. The paging terminal 325 connects callers to the system, accepts and validates messages, and manages the information flow to the system controller 330. The paging terminal 325 also translates subscriber IDs into capcodes and may provide accounting functions.
The system controller 330 handles the queuing, batching, encoding and scheduling of messages received from the paging terminal 325 for delivery to the link network 335. The link network 335 couples the system controller 330 to the transmitter/receiver sites 340 and 350. The link network 335 may be either wired or wireless, and may be switched or packet based.
Since RF bandwidth is a limited resource, the system controller 330 must schedule and route the incoming messages in order to maximize the information flow while minimizing latency.
The messages received from the link network 335 are transmitted by transmitter sites 340 and 350. In general, one-way paging systems will have only a transmitter for communication with a pager device, whereas two-way paging systems will have both a transmitter and receiver.
The transmitter power for the transmitter/receiver sites 340 and 350 may range from less than 100 watts to several hundred watts, depending upon the service area coverage requirements and the regulations applicable to the transmitter.
In the United States, Title 47 of the Federal Code of Regulations (47 CFR) provides the requirements and conditions for commercial mobile radio service providers. Specifically, 47 CFR, Chapter I, parts 20 and 22 provide regulations applicable to paging at low-band VHF. 47 CFR, Chapter I, parts 20 and 22 (Oct. 1, 1999 Edition) are incorporated herein by reference. Paging performed by a system that meets the requirements of 47 CFR, Chapter I, parts 20 and 22 (Oct. 1, 1999 Edition) is defined as statutory paging. A broadcast mode that conforms to the requirements of 47 CFR, Chapter I, parts 20 and 22 (Oct. 1, 1999 Edition) is defined as a statutory paging mode.
In addition to embodiments of the present invention that fall into the scope of statutory paging systems, or paging systems that may be operated in a statutory paging mode, a subset of the statutory parameters (e.g. transmitted power, channel bandwidth, and channel center frequency) may be changed to provide a modified statutory paging system or modified statutory paging mode.
Referring again to FIG. 3, pager 355 and pager 360 are typically single frequency devices tuned to their respective transmitters 340 and 350. Pagers are usually only capable of receiving messages, but may also include various levels of two-way messaging.
Each pager has a unique address (capcode) that is used to select the specific device that is to receive a directed message. A pager may also share a multicast address with one or more other pagers, allowing a single message to be received by several pagers.
Prior Art FIG. 4 shows a paging system 400 with system controller 405 and link network 410 serving non-overlapping zones 450, 455, and 460 using a single frequency. In this system, zones 450, 455, and 460 are sufficiently separated so that a pager is only able to receive a signal from at most one transmitter, regardless of its location. Different messages may be simultaneously transmitted in each zone without interference.
Prior Art FIG. 5 shows a paging system 500 system controller 505 and link network 510 serving zone 550 and overlapping zones 555 and 560. Interference in the overlap area 565 is prevented by using two different frequencies in zones 555 and 560. The two frequencies may be alternated between zone 555 and zone 560 so that pagers tuned to either frequency may receive messages in either zone.
Prior Art FIG. 6 shows a paging system 600 with system controller 605 and link network 610 serving non-overlapping zones 650, 655, 660, and superzone 665, using a single frequency. This system is similar to the system shown in FIG. 4; however, the system controller 605 coordinates the transmitters in zones 650, 655 and 660 so that message delivery is divided into two time periods.
In the first time period used for local service area users, messages are transmitted independently in each of the zones as is done in the system of FIG. 4. In the second time period used for wide area users, the same message is transmitted simultaneously in all three zones. Depending upon the balance of messages between local and wide area users, the transmission time dedicated to the first and second periods can be dynamically adjusted.
RTK positioning systems have been accustomed to using dedicated wireless links for communication between reference stations and rovers. The frequencies at which these links have traditionally been operated are becoming increasingly congested, reducing the areas and times in which the systems can be used freely.
Low-band VHF frequencies that have previously been reserved for applications such as paging have seen a declining utilization as the demand for increased data rate has caused users to migrate to higher frequencies.